Chapter 1
Introduction

In almost every churchyard, you'll find gravestones so old that their inscriptions have disappeared. Over the years, drop after drop of a mild acid has eaten away the stone from which many old gravestones were carved, obliterating the names of those long gone. We know this mild acid as rainwater, formed by the condensation of water vapour containing traces of atmospheric gases like carbon dioxide (CO2) and sulphur dioxide (SO2). It's the gases that make it acid. Rain eats rock by weathering.

Weathering is fundamental to climate change. Over time, it moves mountains. Freezing and thawing cracks new mountain rocks apart. Roots penetrate cracks as plants grow. Rainwater penetrates surfaces, dissolving as it goes. The CO2 in the dissolved products of weathering eventually reaches the sea, where it forms food for plankton, and the seabed, in the remains of dead organisms. Once there, it goes on to form the limestones and hydrocarbons of the future; one day, volcanoes will spew that CO2 back into the atmosphere and the cycle will begin all over again.

The carbon cycle includes the actions of land plants, which extract CO2 from the air by photosynthesis. When plants die, they rot, returning their CO2 to the air. Some are buried, preserving their carbon from that same fate, until heat from the Earth's interior turns them back into CO2, which returns to the air. This natural cycle has been in balance for millions of years. We have disturbed it by burning fossil carbon in the form of coal, oil or gas.

This book is the story of climate change as revealed by the geological record of the past 450 million years (450 Ma). It is a story of curiosity about how the world works and of ingenuity in tackling the almost unimaginably large challenge of understanding climate change. The task is complicated by the erratic nature of the geological record. Geology is like a book whose pages recount tales of the Earth's history. Each copy of this book has some pages missing. Fortunately, the American, African, Asian, Australasian and European editions all miss different pages. Combining them lets us assemble a good picture of how Earth's climate has changed through time. Year by year, the picture becomes clearer, as researchers develop new methods to probe its secrets.

As we explore the evolution of Earth's climate, we will follow the guidance of one of the giants of 18th-century science, Alexander von Humboldt, who wrote in 1788, 'The most important result of research is to recognize oneness in multiplicity, to grasp comprehensively all individual constituents, and to analyze critically the details without being overwhelmed by their massiveness'1. All too often, those who seek to deny the reality of modern climate change ignore his integrative approach to understanding nature by focusing on just one or two aspects where the evidence seems, at the moment, to be less than compelling.

Can the history of Earth's climate tell us anything about how it might evolve if we go on emitting gigatonnes (Gt) of CO2 and other greenhouse gases into the atmosphere? That is the key question behind the title to this book. I wrote it because I have spent most of my career working on past climate change, and it worries me that few of the results of the growing body of research on that topic reach the general public. Even many professional Earth scientists I meet, from both academia and industry, know little of what the most up-to-date Earth science studies tell us about climate change and global warming. For the most part, they have specialised in those aspects of the Earth sciences that were relevant to their careers. Unfortunately, their undoubted expertise in these topics does not prevent some of them from displaying their ignorance of developments in the study of past climate change by trotting out the brainless mantra, 'the climate is always changing'. Well, of course it is, but that ignores the all-important question: Why?

What we really need to know is in what ways the climate has been changing, at what rates, with what regional variability, and in response to what driving forces. With these facts, we can establish with reasonable certainty the natural variability of Earth's climate, and determine how it is most likely to evolve as we pump greenhouse gases into the atmosphere. This book attempts to address these issues in a way that should be readily understandable to anyone with a basic scientific education. It describes a voyage of discovery by scientists obsessed with exposing the deepest secrets of our changing climate through time. I hope that readers will find the tale as fascinating as I found the research that went into it.

The drive to understand climate change is an integral part of the basic human urge to understand our surroundings. As in all fields of science, the knowledge necessary to underpin that understanding accumulates gradually. At first we see dimly, but eventually the subject matter becomes clear. The process is a journey through time, in which each generation makes a contribution. Imagination and creativity play their parts. The road is punctuated by intellectual leaps. Exciting discoveries change its course from time to time. No one person could have discovered in his or her lifetime what we now know about the workings of the climate system. Thousands of scientists have added their pieces to the puzzle. Developing our present picture of how the climate system works has required contributions from an extraordinary range of different scientific disciplines, from astronomy to zoology. The breadth of topics that must be understood in order for us to have a complete picture has made the journey slow, and still makes full understanding of climate change and global warming difficult to grasp for those not committed to serious investigation of a very wide-ranging literature. The pace of advance is relentless, and for many it is difficult to keep up. And yet, as with most fields of scientific enquiry, there is still much to learn - mostly, these days, about progressively finer levels of detail. Uncertainties remain. We will never know everything. But we do know enough to make reasonably confident statements about what is happening now and what is likely to happen next. Looking back at the progress that has been made is like watching a timelapse film of the opening of a flower. Knowledge of the climate system unfolds through time, until we find ourselves at the doorstep of the present day and looking at the future.

While the story of Earth's climate evolution has a great deal to teach us, it is largely ignored in the ongoing debate on global warming. The idea of examining the past in order to discover what the future may hold is not a new one. It was first articulated in 1795 by one of the 'fathers of geology', James Hutton. But it is not something the general public hears much about when it comes to understanding global warming. This book is a wake-up call, introducing the reader to what the geological record tells us.

Information about the climate of the past is referred to as 'palaeoclimate data' (American spelling drops the second 'a'). As it has mushroomed in recent years, it has come to claim more attention from Working Group I of the Intergovernmental Panel on Climate Change (IPCC). The Working Group comprises an international group of scientists, which surveys the published literature every 5 years or so to come up with a view on the current state of climate science. It has been reporting roughly every 5 years since its first report in 1990. Each of its past two reports, in 20072 and 20133, incorporated a chapter on palaeoclimate data. The Working Group's report is referred to as a 'consensus', meaning the broad agreement of the group of scientists who worked on it. Just one chapter in a 1000-page report does not constitute a major review of Earth's climate evolution: the subject deserves a book of its own, and there are several, as you will see from the Appendix to the present book.

The study of past climates used to be the exclusive province of geologists. They would interpret past climate from the character of rocks: coals represented humid climates; polished three-sided pebbles and cross-bedded red-stained sands represented deserts; grooved rocks indicated the passage of glaciers; corals indicated tropical conditions; and so on. Since the 1950s, we have come to rely as well on geochemists using oxygen isotopes and the ratios of elements such as magnesium to calcium (Mg/Ca) to tell us about past ocean temperatures. And in recent years we have come to realise that cores of ice contain detailed records of past climate change, as well as bubbles of fossil air; glaciologists have joined the ranks.

Climate modellers have also contributed. Since the 1950s, our ability to use computers has advanced apace. We now use them not only to process palaeoclimate data and find correlations, but also to run numerical models of past climate systems, testing the results against data from the rock record. Applying numerical models to past climates that were much colder or much warmer than today's has an additional benefit: it helps climate modellers to test the robustness of the models they use to analyse today's climate and to project change into the future. One of my reasons for writing this book is to underscore how research into past climates by both of these research streams, the practical and the theoretical, adds to our confidence in understanding the workings of Earth's climate system and in predicting its likely...